Technical SupportA guide to material selection for rolling bearings
By offering a wide choice of materials and heat treatments for rolling bearings, perfor♦mance and operating life can be optimised, even for the m✘ost demanding industrial applications, says Dr Steve Lacey→, Engineering Manager at Schaeffler UK.
In recent years, there has been a significant incr ease in the demand for rolling bearings that, even under± extreme operating conditions – including lubricant starvatio★n, highly corrosive or high temperature environments – still ≤provide a long operating life and optimum performance.
But selecting a suitable material or heat treatment pr©ocess for rolling bearings often requires expert advi→ce and guidance, normally from the manufacturer of the bearings. Selection depends on the application i©tself and the specific environment in which the bearings will operate. T★his means a number of factors require careful consideration,∏ such as the mechanical, chemical and thermal requirements place¥d on the bearings, as well as lubrication conditions∑, particularly if the application requires dry running bearings.★
There is a key role here for the type of materials used for the various bearing components (i.e. rings, rolling elements, cage, etc.) and how these interact.↑ Some of these materials are industry-recognised standard steels (in rolling bearing φquality), but others may need to be specially developed by the manuf¶acturer or might involve special surface or heat treatment processes to give the materi↕al a certain set of characteristics that are deemed critical for a certain appl≈ication.
Standard rolling bearing steels
For most industrial applications, standard through hardened rolling bearing s§teel (100Cr6) is sufficient. This versatile material i↕s heat treated (martensitic-hardened and tempered at low temperatures), γwhich provides consistently high hardness (between 62 and 64 HRεC) distributed over the circumference and cross-section. ✘The main attributes of this material are high hardness and wear resistance, as well a♥s good resistance to over-rolling (solid particles can produce surface indentations of raceways in< rolling-sliding lubricated contacts).
Depending on the wall thickness of the components, it may be necessary to use a hig§her-grade alloy taking account of hardenability. The material also permits ope§rating temperatures up to 120 deg C, with some grades offering up↑ to 200 deg C. Typical applications include small ↑combustion engines and fan bearings for steel mills.
A slight variation on the above is bainitic hardened 100Cr6 steel. W ith this material, there is a considerable reduction in the risk of crack formation, ×even with surface damage. Typical applications for these bearings include wheelset bear★ings for bucket wheel excavators and trams.
Case hardened steel (St4) in rolling bearing quality has good forming characteristics and provide∑s high hardness and wear resistance, as well as good resistance to over-rolling. This> material is well suited to small components that are manufactured in high volumes, such as universal joints, drawn cup needl®e roller bearings with closed ends and swing arm bearings.
If there is a high risk of particle over-rolling and poor lubricating conditions (≈e.g. refrigerating compressors, gearbox bearings for ♦construction/agricultural machinery), carbonitrided steel (100CrMnSi6-4) is the preferred choicβe. This material is heat treated to provide excellent dimensional stability and hardness up t€o 66 HRC in the outer functional layer by using increased residual austenite content. The result i's a material that provides higher resistance to surface damage and high wear <resistance, leading to extended bearing life in poor lubrication co<nditions, especially where there is a high risk of contamination.
Wind turbines & trucks
Used primarily for medium and large sized rolling bearings, including wind ✘turbine main rotor bearings and wheelset bearings for heavy-duty trucks, case hardened (carburised and martensitic haπrdened) steel offers hardness up to 64 HRC in the f orm of a hard surface layer with a tough core. This treatment providεes residual compressive stresses in the surface layer, providing incre ased fracture resistance under shock type loads or surface damage, as well as reduced crack formati"on with surface damage.
Nitrogen alloyed chromium steel (e.g. Schaeffler’sCronidur® 30) is a material that is ma×rtensitic hardened and tempered at low (or possibly high) temperature. This mat↔erial provides excellent corrosion resistance, particula♣rly against aggressive chemicals and acids. Operating temperatures are typically up to 150 deg C. Main areas of application are bearings that are subjected to high₹ loads, lubricant starvation, dry running, media lubrication and cor≠rosive environments. Examples include spindle bearings for machine tools and bearin£gs for process pumps.
Corrosion resistant, case hardened steel (surface layer nitriding) provides rol∞ling bearings with even better resistance to corrosion and significantly improves resistance≈ to over-rolling. Schaeffler’sCronitect® steel, for example, has found numerous a₹pplications in food processing (e.g. bottle filling p☆lants), as well as in sports and fitness applications (e.g. inline roller sφkates and gym equipment).
Special Materials
Often bearings manufacturers are required to provide special materials and heat t¶reatments for certain applications or industry sectors. The aeros→pace sector, for example, specifies M50 steel (martensitic hardened☆) for many applications such as main shaft bearings and turbine rotor bearings on aλircraft engines. This material provides excellent thermal stability (up to 400ε deg C) and increased toughness, even at high bearing operating speγeds. Offering even higher hardness (up to 70 HRC) are duplex hardened M50NiL s✘teels. These are used if the bearings are subjected to high loads, very high rota↓tional speeds and if a high tolerance to damage is required.
Where current insulation and high wear resistance are important considerations, oxi$de-based ceramics (e.g. zirconium oxide ZrO2) are ideal. These sinte✔red materials can be used in temperatures up to 600 deg C, with typical hardness ratings in t¶he region of 1400 HV and a density between Si3N4 and steel. The material is normally only use₩d for the rolling elements that are in contact with steel-based bearing ri₽ngs. Main areas of application are where the bearinαgs are subjected to low loads (hybrid bearings), lubricant starvation, dry r'unning, media lubrication and corrosive environments (e.g. electric motors for car windows)↓.
In certain food processing applications and for ma®chine tool spindle bearings, silicon nitride-based ceramic rolling εelements can be specified. The technical advantages∑ of this material include its reduced mass and ability to withstand higher operating temperatures (∏up to 700 deg C), as well as high speed capacity, current insulation, very high wear resistance,σ reduced friction and increased grease operating life.
Steel rolling mill bearings and some marine propulsion s☆ystems use case hardened rolling bearing steel (to DIN EN 10084). Examples here includeδ 17MnCr5 and 17CrNiMo7-6. These materials are specified if the bearing©s are subjected to high loads combined with poor lubrication and when there is a high risk ≤of particle over-rolling.